Electronic differential lock assembly

ABSTRACT

An electronic differential lock assembly includes a shift collar that is movable in response to an electronic signal from an unlocked position where axle shaft speed differentiation under predetermined conditions is permitted to a locked position where a pair of axle shafts is fixed for rotation together. Speed differentiation is provided by a differential that includes a differential gear assembly supported within a differential case. A coil surrounds the shift collar and is selectively energized to move the shift collar from the unlocked position to the locked position. The shift collar is splined to one of the axle shafts and is selectively splined to the differential case to lock the axle shafts together. The electronic differential lock assembly includes a return spring that automatically disengages the shift collar from the differential case once the coil is no longer energized.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/543,080 filed on Jul. 21, 2005, which is a 371 of PCT/IT03/00044filed on Jan. 31, 2003.

BACKGROUND OF THE INVENTION

This invention relates to a differential locking assembly that utilizesan electronic coil to actuate a differential lock shift collar.

Vehicle drive axles typically include a pair of axle shafts for drivingvehicle wheels. The drive axle uses a differential to control inputspeed and torque to the axle shafts. Under ideal conditions, when thevehicle is driven along a straight path, the wheels will be turning atapproximately the same speed and the torque will be equally splitbetween both wheels. When the vehicle negotiates a turn, the outer wheelmust travel over a greater distance than the inner wheel. Thedifferential allows the inner wheel to turn at a slower speed than theouter wheel as the vehicle turns.

Power is transmitted from a vehicle drive shaft to a pinion gear that isin constant mesh with a differential ring gear. The ring gear is boltedto a differential housing or case that turns with the ring gear. Adifferential spider having four (4) support shafts orientated in theshape of a cross, has four (4) differential pinion gears. One piniongear is supported for rotation on each support shaft. Power istransferred from the differential housing to side gears that are splinedto the axle shafts. The side gears are in constant mesh with the sidedifferential pinion gears. The outer ends of the axle shafts are boltedto the brake drum hubs to which the wheels are also bolted.

When the vehicle is driven in a straight path, the ring gear,differential housing, spider, and differential pinion gears all rotateas one unit to transfer power to the axle shafts. There is no relativemovement between the differential pinion gears and the side gears. Whenthe vehicle turns, the differential pinion gears rotate on theirrespective shafts to speed up the rotation of one axle shaft whileslowing the rotation of the other axle shaft.

Often the differential includes a differential locking mechanism. Whenthere are poor road conditions, i.e., slippery or rough surfaced roads,the locking mechanism allows maximum wheel and tire traction forimproved control. If the differential does not have a locking mechanismand one tire is on ice, all of the torque and speed will be transferredto the wheel on ice. Thus, the tire just spins on the ice and thevehicle is prohibited from traveling forward. A locking mechanism allowsthe axle shafts to rotate at the same speed while transferring allavailable torque to the tire not on the ice. If the tractive effort atthis tire is sufficient, the vehicle can be moved off of the ice. Whenthe differential is locked, power is transmitted through the lockeddifferential housing, gearing, and axle shafts rather than through thedifferential gearing only.

One type of differential locking mechanism is comprised of an airactuated shift collar that locks the differential housing to the axleshafts. An air operated shift fork cooperates with the shift collar toengage or disengage the locking mechanism via a driver-controlledswitch. In this configuration, one of the axle shafts has two sets ofsplines. An inner set of splines, closest to the differential, isengaged with one differential side gear, while an outer set of splinescooperates with the shift collar. The shift collar, although engagedwith the outer splines at this time, is not engaged with thedifferential casing, thus the outer splines will rotate at the samespeed as this side gear when the main differential is in an unlocked ordisengaged position allowing the main differential to operate in anormal manner.

When the driver-controlled switch is activated, air pressure causes ashift linkage to move the shift collar towards the differential. Thisallows the collar to engage with the differential casing, as well asremaining engaged with the axle shaft outer splines. Power transferthrough the differential is now achieved through the locked differentialcasing, gearing, and both axle shafts together, rather than through thedifferential gearing alone.

Some disadvantages with the air actuation method are the significantnumber of components that are required, leakage, and component wear. Thesignificant number of components that are required to operate thissystem increase assembly time and drive up the overall system cost.Requiring an air connect to actuate the system introduces possible airleaks to the system, which can lead to inadequate system performance.Further, the differential gearing, axle shafts, and shift collaroperates in an oil-lubed environment and the additional components forthe air actuation method increase the risk of oil leaks in the system.Further, repeated engagements and disengagements between the shift forkand shift collar, especially if engaged at high wheel speeds, can leadto premature component wear as well as introducing premature wear ontorelated components such as the differential gearing.

Thus, it is desirable to have a simplified actuating mechanism for adifferential lock that reduces the overall number of components,operates more efficiently, and is more cost effective, as well asovercoming the other above-mentioned deficiencies with the prior art.

SUMMARY OF THE INVENTION

A differential locking mechanism includes a clutch or shift collar thatis movable in response to an electronic signal from an unlocked positionwhere axle shaft speed differentiation under predetermined conditions ispermitted to a locked position where axle shaft speed differentiation isnot permitted. The differential locking mechanism is incorporated into adrive axle assembly that includes a differential having a differentialgear assembly supported within a differential case. A pair of axleshafts are driven by the differential gear assembly, which receivesdriving input from a ring and pinion gear set coupled to a drive shaft.An electronic actuator generates an electronic signal to move the shiftcollar from the unlocked position to the locked position.

Preferably, the electronic actuator comprises a coil that surrounds aportion of the shift collar. The coil is selectively energized inresponse to an input command to lock the differential. When the coil isenergized, the shift collar is moved in an inboard direction, toward thedifferential, to engage the differential case, which locks thedifferential. A resilient member, such as a spring, for example,automatically disengages the shift collar from the differential case andmoves the shift collar back to the unlocked position when the coil isnot energized.

In one disclosed embodiment, the shift collar includes an inboard endhaving a splined surface for selective engagement with a mating splinedsurface on the differential case and an outboard end, which issurrounded by the coil. Preferably, the outboard end is smaller indiameter than the inboard end. The outboard end includes a splined borein constant engagement with a mating splined surface on one of the axleshafts. A washer is mounted to the outboard end of the shift collar. Theresilient member reacts between the washer and the coil to return theshift collar to the unlocked position.

The method for controlling the differential lock assembly includes thefollowing steps: energizing the coil surrounding the shift collar; andin response to the coil being energized, moving the shift collar fromthe unlocked position where speed differentiation between the pair axleshafts is permitted under predetermined conditions to a locked positionwhere both of the axle shafts rotate at a common speed by fixing theshift collar to the differential case.

The subject invention provides a simplified and effective differentiallock that significantly reduces the number of components fromtraditional designs, reduces assembly time, and reduces cost. These andother features of the present invention can be best understood from thefollowing specifications and drawings, the following of which is a briefdescription.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vehicle powertrain with a traditionaldifferential locking mechanism.

FIG. 2 is an exploded view of a traditional differential for a driveaxle.

FIG. 3 is a cross-sectional view, partially broken away, of atraditional differential locking mechanism showing engaged anddisengaged positions.

FIG. 4 is an exploded view of the differential locking mechanism of FIG.3.

FIG. 5 is a schematic view of a vehicle powertrain with a drive axlehaving a differential incorporating the subject invention.

FIG. 6 is an exploded view of the inventive differential lockingmechanism.

FIG. 7 is a cross-sectional view, partially broken away, of adifferential incorporating the differential locking mechanism of FIG. 6shown in a disengaged position.

FIG. 8 is a view similar to that of FIG. 7 but shown in an engagedposition.

FIG. 9 is cross-sectional view of the shift collar shown in FIG. 6.

FIG. 10 is an overhead view of the washer shown in FIG. 6

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A traditional powertrain with an air-actuated differential lockingmechanism is shown generally at 12 in FIG. 1. The powertrain 12 includesan engine 14, transmission 16, and drive shaft 18 that is coupled to adrive axle 20. The drive axle 20 includes a carrier 22 positioned withinan axle housing 24 as known. The carrier 22 includes a pinion gear 26,operably coupled to the drive shaft 18, and which is in drivingengagement with a ring gear 28. The ring gear 28 drives a differential30, which in turn drives a pair of axle shafts 32 that drive the vehiclewheels (not shown).

The differential 30 includes a differential locking mechanism, showngenerally at 34, which is actuated via an air supply connection 36. Thedifferential locking mechanism 34 is movable between an unlockedposition where axle shaft speed differentiation is permitted and alocked position where the axle shafts are locked together to rotate at acommon speed. The differential locking mechanism 34 includes a shiftcollar 38 and an air actuated shift fork 40.

As shown in FIG. 2, the differential 30 includes a differential gearassembly, shown generally at 42, and a two-piece differential housingcase 44. The two-piece differential case 44 includes a flange case half44 a that is attached to the ring gear 28 with a plurality of fasteners(not shown) such that the flange case half 44 a and ring gear 28 rotatetogether as a single unit. The differential gear assembly 42 includes adifferential spider 46 that has four (4) support shafts 48 orientated inthe shape of a cross and has four (4) differential pinion gears 50. Onedifferential pinion gear 50 is supported for rotation on each supportshaft 48. The second piece of the two-piece differential case 44comprises a plain case half 44 b that includes four (4) slots 52 (onlythree (3) are shown). Each of the support shafts 48 is received withinone of the slots 52 and the plain case half 44 b is attached to theflange case half 44 a with a plurality of fasteners (not shown). Thedifferential gear assembly 42 also includes a pair of side gears 54 thatare in meshing engagement with the differential pinion gears 50. Oneside gear 54 is splined to each axle shaft 32.

Power is transferred from the pinion gear 26 to the differential case 44to the axle shafts 32 via the side gears 54. The side gears 54 are inconstant mesh with the differential pinion gears 50. When the vehicle isdriven in a straight path, the ring gear 28, differential case 44,differential spider 46, and differential pinion gears 50 all rotate asone unit to transfer power to the axle shafts 32 via the side gears 54.There is no relative movement between the differential pinion gears 50and the side gears 54. When the vehicle turns, the differential piniongears 50 rotate on their respective shafts 48 to speed up the rotationof one axle shaft 32 while slowing the rotation of the other axle shaft32.

For adverse road conditions, the differential locking mechanism 34 isactuated to provide maximum wheel and tire traction for improvedcontrol. The locking mechanism 34 allows the axle shafts 32 to rotate atthe same speed while transferring all available torque to the tiresupported on the higher friction surface. For example if the tires atone end of the axle 20 were supported on an icy surface with the tire onthe opposite end of the axle being supported on pavement, all availabletorque would be transferred to the tires on the pavement. If thetractive effort at these tires is sufficient, the vehicle can be movedoff of the ice. When the differential 30 is locked, power is transmittedthrough the locked differential case 44, differential gear assembly 42,and axle shafts 32 rather than through the differential gear assembly 42only.

As described above and as shown in greater detail in FIG. 3, thetraditional differential locking mechanism 34 includes the air actuatedshift collar 38 that locks the differential case 44 to the axle shafts32. The air operated shift fork 40 cooperates with the shift collar 38to engage or disengage the locking mechanism 34 via a driver controlledswitch (not shown). A push rod and spring assembly, shown generally at56, are activated pneumatically via an air connect 58.

The shift collar 38 is locked to the differential case 44 in thefollowing manner. One of the axle shafts 32 includes an inboard set ofsplines 60, closest to the differential 30, and an outboard set ofsplines 62. The inboard set of splines 60 are engaged with onedifferential side gear 54, while the outboard set of splines 62cooperate with the shift collar 38. The shift collar 38, althoughengaged with the outboard set of splines 62 in an unlocked position(shown in solid lines in FIG. 3), is not engaged with the differentialcase 44, thus the outboard set of splines 62 will rotate at the samespeed as the respective side gear 54 when the differential 30 is in anunlocked or disengaged position allowing the main differential tooperate in a normal manner.

When the driver controlled switch is activated, air pressure causes thepush rod and spring assembly 56 to move the shift collar 38 towards thedifferential 30 with the shift fork 40. This allows the shift collar 38to engage with the differential case 44, while still remaining engagedwith the axle shaft outboard set of splines 62 to achieve the lockedposition (shown in dashed lines in FIG. 3). Power transfer through thedifferential 30 is now achieved through the locked differential case 44,differential gear assembly 42, and both axle shafts 32 together, ratherthan through the differential gear assembly 42 alone.

An exploded view of the traditional differential locking mechanism 34showing the shift fork 40, shift collar 38, and push rod and springassembly 56 is shown in FIG. 4. The push rod and spring assembly 56includes a push rod 64 with a piston 66, o-ring 68, and gasket 70supported on one end. Four (4) cap screws 72 and four (4) associatedwashers 74 (only one set is shown) attach an air cylinder cover 76 withassociated plug 78 to the gasket 70. A spring 80 surrounds an oppositeend of the push rod 64 and reacts between the piston 66 and shift fork40. A pair of pins 82 cooperate with the shift fork 40 to maintainproper orientation. A microswitch 84 and associated attachment nut 86are also included in the differential locking mechanism 34 to monitorand inform the driver of the status of the differential 30. Thus, overtwenty (20) separate components are required to operate a traditionaldifferential locking mechanism.

This significant number of components in addition to increasing materialcosts makes assembly more difficult, which increases assembly time andalso adds to the overall cost. Further, requiring an air connect toactuate the system introduces possible air leaks to the system, whichcan lead to inadequate system performance. Also, the differential gearassembly 42 operates in an oil-lubed environment and the air actuationincreases the risk of oil leaks in the system. Further, repeatedengagement and disengagements between the shift fork 40 and shift collar38, especially if engaged at high speeds, can lead to prematurecomponent wear as well as introducing premature wear onto relatedcomponents such as the differential gear assembly 42.

The subject invention provides an improved locking mechanism thatsignificantly reduces the number of components and reduces overall costand assembly time. A powertrain incorporating the subject invention isshown generally at 100 in FIG. 5. The powertrain 100 includes an engine102, transmission 104, and drive shaft 106 that is coupled to a driveaxle 108. The drive axle 108 includes a carrier 110 positioned within anaxle housing 111 as known. It should be understood that the axle housing111 includes housing portions that substantially enclose the axle shafts118 and the carrier 110 within the housing 111. Additionally, thehousing 111 can be formed as a single piece or can be formed frommultiple pieces that are attached together to form the complete axle andcarrier housing.

The carrier 110 includes a pinion gear 112, operably coupled to thedrive shaft 106, and which is in driving engagement with a ring gear114. The ring gear 114 drives a differential 116, which in turn drive apair of axle shafts 118 that drive the vehicle wheels (not shown). Thepinion gear 112 defines a longitudinal axis 120 and the axle shafts 118define a lateral axis 122 that is transverse to the longitudinal axis120. It should be understood that the differential 116 includes similarcomponents and operates in a similar manner to that described withregard to differential 30 above and shown in FIG. 2.

The differential 116 includes a differential locking mechanism, showngenerally at 124, which includes a shift collar 126 that iselectronically actuated via a coil 128. The coil 128 is comprised of acoiled wire that surrounds a portion of the shift collar 126. The coil128 is connected to a power source 130, such as a battery, thatselectively energizes the coil 128 via an electronic signal 132. Whenthe coil 128 is energized via an input command 134 from a driver orother input source, the shift collar 126 acts as an electromagnet andcan be moved from an unlocked position to a locked position. Theoperation of electromagnets is well known in the art and will not bediscussed in detail.

An exploded view of the differential locking mechanism 124 is shown inFIG. 6. The coil 128 includes mount portions 136 that cooperate withfastener and nut assemblies 138 to attach the coil 128 to a portion ofthe carrier or axle housing 111. The coil 128 forms a bore 140 in whichone end of the shift collar 126 is received. A resilient return member142, such as a coil spring, spring discs, or other similar mechanism,cooperates with a washer 144 to return the shift collar 126 to anunlocked position once the coil 128 is no longer energized. Amicroswitch 146 and associated attachment nut 148 are also included inthe differential locking mechanism 124 to monitor and inform the driverof the status of the differential 116. Thus, the number of componentsfor the differential locking mechanism 124 are reduced by over half whencompared to the traditional air actuated design.

FIG. 7 shows the differential locking mechanism 124 in the unlocked ordisengaged position with the coil 128 in an unenergized state. The shiftcollar 126, splined to the axle shaft 118 in a manner similar to thatdescribed above, is positioned outboard of and is disengaged from thedifferential case 44.

FIG. 8 shows the differential locking mechanism 124 in the engaged orlocked position. In this position, the coil 128 is energized via thepower source 130, which moves the shift collar 126 linearly in aninboard direction along the lateral axis 122 until a spline attachment150 is achieved with the differential case 44.

The axle shaft 118 includes an inboard set of splines 152, closest tothe differential 116, and an outboard set of splines 154. The inboardset of splines 152 are engaged with one differential side gear 54, whilethe outboard set of splines 154 cooperate with the shift collar 126. Theshift collar 126, although engaged with the outboard set of splines 154in an unlocked position (shown in solid lines in FIG. 3), is not engagedwith the differential case 44, thus the outboard set of splines 62 willrotate at the same speed as the respective side gear 54 when thedifferential 116 is in an unlocked or disengaged position allowing themain differential to operate in a normal manner. When the input command134 is issued, the power source 130 is energized, which causes the shiftcollar 126 to move towards the differential 116. This allows the shiftcollar 126 to engage with the differential case 44, while stillremaining engaged with the axle shaft outboard set of splines 154 toachieve the locked position (shown in dashed lines in FIG. 3). Powertransfer through the differential 116 is now achieved through the lockeddifferential case 44, differential gear assembly 42, and both axleshafts 118 together, rather than through the differential gear assembly42 alone.

The shift collar 126 is shown in greater detail in FIG. 9. The shiftcollar 126 includes an inboard end 160 and an outboard end 162 that issmaller in diameter than the inboard end 160. A central bore 164 extendsthrough the shift collar 126 and receives the axle shaft 118. Thecentral bore 164 includes a splined surface 166 that mates with theoutboard set of splines 154 of the axle shaft 118. The inboard end 160includes a splined surface 168 that engages a mating splined surface 170on the differential case 44 indicated at 150 (see FIGS. 7 and 8).

The washer 144 is mounted to the outboard end 162 of the shift collar126. The washer 144, shown in greater detail in FIG. 10, includes acentral bore 172 that is received over the 126. A plurality of slots 174are formed about the circumference of the washer 144 which is greater indiameter than the outboard end 162 but smaller in diameter than theinboard end 160. The resilient return member 142 reacts between thewasher 144 and the coil 128 to return the shift collar 126 to theunlocked position.

The method for controlling the differential locking mechanism 124 forthe drive axle 108 includes the steps of energizing the coil 128, whichsurrounds a portion of the outboard end 162 of the shift collar 126; andin response to energizing the coil 128, moving the shift collar 126 fromthe unlocked position where speed differentiation between the pair axleshafts 118 is permitted under predetermined conditions to a lockedposition where both of the axle shafts 118 rotate at a common speed byfixing the shift collar 126 to the differential case 44. Additionally,the shift collar 126 is automatically returned to the unlocked position,i.e. is disengaged from the differential case 44, when the power supply130 is cut from the coil 128.

The subject invention provides a simplified and effective differentiallock that significantly reduces the number of components fromtraditional designs, reduces assembly time, and reduces cost. Although apreferred embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A differential locking mechanism for a drive axle comprising: adifferential including a differential gear assembly supported within adifferential case having a first case half that is to support a ringgear and a second case half that supports a differential spider; a pairof axle shafts driven by said differential gear assembly for rotationabout an axis; a shift collar movable between an unlocked position wherespeed differentiation between axle shafts is permitted and a lockedposition wherein said shift collar directly engages said second casehalf of said differential case such that said differential case, saidshift collar, and said pair of axle shafts are fixed for rotationtogether; and an electronic actuator responsive to an electronic signalto move said shift collar from said unlocked position to said lockedposition.
 2. The differential locking mechanism as set forth in claim 1wherein said electronic actuator includes a coil mounted to a fixed axlecomponent and surrounding said shift collar wherein said electronicsignal powers said coil to move said shift collar.
 3. The differentiallocking mechanism as set forth in claim 2 wherein said fixed axlecomponent comprises a non-rotating housing.
 4. The differential lockingmechanism as set forth in claim 1 wherein said electronic actuatorincludes a coil that is responsive to said electronic signal to movesaid shift collar to said locked position and a resilient member forautomatically returning said shift collar to said unlocked position whensaid coil is not powered, said coil and said resilient membersurrounding said shift collar.
 5. The differential locking mechanism asset forth in claim 4 wherein said resilient member reacts directlybetween said coil and an outboard end of said shift collar.
 6. Thedifferential locking mechanism as set forth in claim 5 including awasher fixed to said outboard end for reacting with said resilientmember.
 7. The differential locking mechanism as set forth in claim 2wherein said shift collar includes an inboard end having a splinedsurface and an outboard end for supporting a resilient member, saidinboard end having a greater diameter than said outboard end.
 8. Thedifferential locking mechanism as set forth in claim 7 wherein said coildefines a central bore surrounding said shift collar at said outboardend, said shift collar moving in an inboard direction in response tosaid coil being powered via said electronic signal such that saidsplined surface of said inboard end engages a mating splined surfaceformed on an end of said second case half of said differential case suchthat said differential case and said pair of axle shafts are lockedtogether for rotation about said axis.
 9. A drive axle assembly with alocking differential comprising: a driving input defining a longitudinalaxis; a carrier including a pinion gear driven by said driving input anda ring gear in meshing engagement with said pinion gear; a differentialincluding a differential gear assembly supported by a differential casethat has a first case half and a second case half that cooperate tosurround said differential gear assembly and wherein said ring gear isattached to said first case half to drive said differential gearassembly; a pair of axle shafts driven by said differential gearassembly for rotation about a lateral axis, said lateral axis beingtransverse to said longitudinal axis; and a locking mechanism includinga shift collar and an electronic actuator for controlling movement ofsaid shift collar wherein said shift collar is movable between anunlocked position where speed differentiation between axle shafts ispermitted and a locked position wherein said shift collar is moved intolocking engagement with said second case half in response to anelectronic signal such that said differential case, said shift collarand said pair of axle shafts are fixed for rotation together about saidlateral axis .
 10. The drive axle assembly as set forth in claim 9wherein said electronic actuator comprises a coil surrounding said shiftcollar wherein said electronic signal powers said coil to move saidshift collar.
 11. The drive axle assembly as set forth in claim 10including an axle housing for substantially enclosing said carrier andsaid pair of axle shafts wherein said coil is supported by said axlehousing.
 12. The drive axle assembly as set forth in claim 11 whereinsaid shift collar includes an inboard end having a splined surface andan outboard end, said inboard end having a greater diameter than saidoutboard end and wherein said coil defines a central bore surroundingsaid shift collar at said outboard end, said shift collar moving in aninboard direction in response to said coil being powered via saidelectronic signal such that said splined surface of said inboard endengages a mating splined surface formed on said differential case suchthat said differential case is locked to said pair of axle shafts. 13.The drive axle assembly as set forth in claim 9 wherein said electronicactuator comprises a coil responsive to said electronic signal to movesaid shift collar and a resilient member that returns said shift collarto said unlocked position when said coil is not powered, said coil andsaid resilient member surrounding said shift collar.
 14. The drive axleassembly as set forth in claim 13 wherein said coil is fixed to ahousing.
 15. The drive axle assembly as set forth in claim 9 whereinsaid differential gear assembly includes a differential spider that isreceived within slots formed in said second case half.
 16. A method forcontrolling a differential lock assembly for a drive axle comprising thesteps of: (a) providing a differential for driving a pair of axleshafts, the differential including a differential gear assemblysupported within a differential case, and a shift collar for selectiveengagement with the differential case wherein the differential caseincludes a first case half that is to support a ring gear and a secondcase half that supports the differential gear assembly; (b) energizing acoil surrounding the shift collar; and (c) in response to step (b)moving the shift collar from an unlocked position where speeddifferentiation between the pair axle shafts is permitted underpredetermined conditions to a locked position where the axle shaftsrotate at a common speed by fixing the shift collar to the second casehalf.
 17. The method as set forth in claim 16 including the step ofautomatically returning the shift collar to the unlocked position whenthe coil is not energized.
 18. The method as set forth in claim 16including the step of mounting a differential spider of the differentialgear assembly within slots formed in the second case half.
 19. Themethod as set forth in claim 16 including providing a resilient memberto return the shift collar to the unlocked position and mounting thecoil and the resilient member to surround the shift collar.
 20. Themethod as set forth in claim 16 including mounting the coil to anon-movable housing structure in the drive axle.